"Something has gone in us, not in our attitude, but in our confidence," Arsène Wenger said, thinking back to Tuesday night's Champions League defeat in Barcelona, and groping for explanations after yet another letdown. The question now: how many disappointments can a team take before they lose their faith?

The most optimistic spin on Arsenal crashing out of three competitions in 13 days is that it leaves them free to win the Premier League for the first time since 2004. But to think that way the dedicated Gunner will have to ignore the psychological fallout and disregard the signs that say Arsenal no longer believe they can win big games, however hard they try.

"I believe that subconsciously the disappointment of Tuesday night has played a part in today's game," Wenger said. As ever he faced the music, took the questions. He asks us now to believe these setbacks can be overcome, the damage repaired. In London, Barcelona and now Manchester chunks of Arsenal's season lay strewn around.

Sunday 1 May should tell us. On that day United motor to the Emirates for a game that provides a chance for both sides to land an uppercut. Wenger said: "I believe it will strengthen our resolve at home, and if the Man United game is decisive I believe we can win it with our home games."

These are strong declarations, from a tough manager, but they conflict with our sense of what is happening. With Wayne Rooney's headed goal just after the interval, Arsenal's campaign just seemed to evaporate. All around Old Trafford you could feel a thing of promise turn to savage disillusionment.

The vortex has been particularly merciless. First they lost the Carling Cup final to Birmingham City after a late defensive howler (praise is due to Birmingham, too), then came Barcelona in the Champions League and the night of a thousand agonies.

Against that background a trip to Manchester United for a FA Cup sixth-round tie might have felt like falling into the arms of an old enemy who you respect and loathe in equal measure: a cosy domestic arrangement, especially as United were bound to field a weakened side, with Tuesday's Champions League second leg against Marseille so close. It must have felt like a bit of a refuge, where the dream of a Premier League-FA Cup Double could be revived, and the unpleasant memories of Wembley and the Camp Nou fed through the mental shredder.

Battered by recent disappointments, the two imperial powers were in regrouping mode. United were ahead after 28 minutes through Fábio da Silva and extended that lead with Rooney's close-range header. "Efficient", Wenger called them. Edwin van der Sar, in the United goal, was much better than that. He was majestic.

Telepathy has always been a feature of the best football teams, but just to be sure Manchester United used twins to achieve their breakthrough. Rafael and Fábio, mirror images on either wing, worked with one mind to concoct the opening goal in a game in which 11 of the 20 outfield starters were defenders.

If full-backs can attack like this, who needs wingers? In a United side of seven defenders Rafael da Silva filled the David Beckham/Cristiano Ronaldo berth on the right and Fábio took the role so often filled by Ryan Giggs. On the teamsheet it looked a patching-up exercise. But then Fábio worked an opening on the right, Rooney crossed beautifully, Javier Hernández headed hard and low and Manuel Almunia palmed his save into Fábio's path.

With his work complete Fábio gave way at half-time to the returning Antonio Valencia, the kind of tough, thrusting player United will need to arrest a run of three defeats in five league games. Giggs, too, was summoned to relieve a Da Silva twin, this time Rafael. A familiar scenario kicked in: Arsenal working the ball and pressing outside United's box, the home team waiting for the breakout. "Sterile domination?" There were certainly a lot of Arsenal passes. And they all came to nought.

Desperate, Wenger sent on Tomas Rosicky and Aaron Ramsey for Abou Diaby and Andrey Arshavin. Marouane Chamakh had already replaced Denílson. With so much attacking talent in yellow the mind span back to Holland finishing a qualifier against the Republic of Ireland with four centre-forwards and still coming up empty.

The final anguish for Arsenal was to see one of their most improved defenders, Johan Djourou, removed by stretcher. His season is over. No inquest on Arsenal's season can begin until the Premier League trophy is handed over. To win the championship would heal the pain of the last fortnight instantly. As a snap judgment we can say they lack only a few buyable ingredients for success at home and abroad, though one is mental rigour. They are where they have been since 2005: close.

These two teams will stay locked together in the title race. Neither looks equipped to burn the other off. United advance on three fronts, Arsenal are confined to one. All thought now can be telescoped into one ambition: the championship. Time to stop reaching. Time to grasp.